The invention relates to the general field of atomic force microscopy with particular reference to performing scanning thermal microscopy on specimens that are conducting electricity.
Scanning Thermal Microscopy (SThM) is a technique that uses the specimen""s thermal conductivity as a contrast mechanism in imaging microscopic features. The temperature sensing probe in Atomic Force Microscopy (AFM) can be used for semiconductor material and device study such as locating hot spots created by short circuit defects in the sub micron regime.
Commercial SThMs use a miniature thermal resistor positioned at the end of a cantilever. If a small current is passed through the resistor, and the resistance is measured as the tip is scanned over the surface, a local temperature map of the specimen is produced based on the resistance changes. If, on the other hand, a large current is passed and the resistor temperature rises significantly above that of the specimen, the probe detects local changes in the local thermal conductivity of the sample. In the latter mode of operation, the thermal conductivity of the specimen, as presented at the surface, is an aggregate of any thermal conductivity variations down into the specimen. Changes in composition below the specimen surface will therefore produce a feature in the thermal map.
One of the limitations to current usage of the above probe is that if one needs to obtain the thermal map of an electrically biased specimen, the conductive parts of the specimen must be passivated to prevent excessive current leakage between the tip and the conductive sample. This is illustrated in FIG. 1 where microtip 12, coated with sensing layer 13, is seen to be located near the end of cantilever beam 11. If a current is being passed through 13 and/or through specimen layer 15, leakage, shown schematically as arrows 14, will occur between probe and specimen. The prior art solution to this has been to coat the specimen with a layer of insulation. In many cases, however, this may be difficult and/or undesirable to do.
Additionally, commercial resistive probes are often prone to short circuiting between the conductors should any conductive contaminants end up between the conductive leads that connect to the tips. This is illustrated in FIG. 2 where cantilever beam 11 is seen extending out from one end of insulating substrate 21. After fabrication of the sensing tip (as in FIG.1, for example), the substrate was coated with a conductive material which was then formed into connecting leads 22 and 23 by laser machining trench 24 through the metal down to the substrate level. Since said trench is only about 25 microns wide, a particle of conductive material, such as 25, can, if it bridges trench 24, short circuit the leads 22 and 23.
The present invention enables SThM to be performed directly on electrically biased conductive samples. In addition, it minimizes the probability of its electrical conductors being shorted to each other by conductive contaminants.
A routine search of the prior art was performed and the following references of interest were found: Luo et al. in J. Vac. Sci. Technol. B 15(2) 349-359 discuss manufacturing techniques, some of which were used when the first embodiment of the present invention was first reduced to practice. U.S. Pat. No. 5,969,238 (Fischer) shows a thermoelectric probe tip process. U.S. Pat. No. 5,581,083 (Majumdar et al.) shows a sensor and tip for a scanning thermal microscopy, and U.S. Pat. No 5,811,802 (Gamble) shows a scanning microscope. Pylekki et al. (U.S. Pat. No. 5,441,343) show a thermal sensing scanning probe microscope using resistive sensors while Aslam et al. (U.S. Pat. No 5,488,350) show diamond film structures in different patterns for conducting, generating, and/or absorbing thermal energy.
It has been an object of the present invention to provide a scanning thermal microscope probe that may be used to scan a specimen through which an electric current is passing.
Another object of the invention has been that said probe be of the thermocouple type.
A further object of the invention has been that said probe be of the resistance thermometer type.
A still further object has been to provide processes for manufacturing of said novel probe types.
These objects have been achieved by coating the tips of both the above probe types with a layer of insulation that is also a good thermal conductor. This allows the probes to be used to scan specimens in which an electric current is passing without the presence of a leakage current between probe and specimen.